Only, it seems, out of politeness when replying to others who had misused the term. Otherwise, he commented to Rayner Unwin that trilogy 'is not really accurate' when applied to The Lord of the Rings (Letters, p. 184), and to Houghton Mifflin that 'the book is not of course a "trilogy"' (Letters, p. 221). Christina and I have also run across a similar plea by Tolkien in the Houghton Mifflin archive, that the publisher please not use 'trilogy' in a Lord of the Rings jacket blurb.